In a typical cellular network, also referred to as a wireless communication system, a radio communication system or a communications system, a User Equipment (UE), communicates via a Radio Access Network (RAN) to one or more Core Networks (CNs).
A user equipment is a device that may access services offered by an operator's core network and services outside the operator's network to which the operator's radio access network and core network provide access. The user equipment may be any device, mobile or stationary, enabled to communicate over a radio channel in a communications network, for instance but not limited to e.g. mobile phone, smart phone, tablet computer, sensors, meters, vehicles, household appliances, medical appliances, media players, cameras, Machine to Machine (M2M) device, or any type of consumer electronic, for instance but not limited to television, radio, lighting arrangements, tablet computer, laptop or Personal Computer (PC). The user equipment may be portable, pocket storable, hand held, computer comprised or vehicle mounted mobile devices, enabled to communicate voice and/or data, via the radio access network, with another entity.
The user equipment is enabled to communicate wirelessly in the communications system. The communication may be performed e.g. between two UEs, between a UE and a regular telephone and/or between the UE and a server via the radio access network and possibly one or more core networks, comprised within the communications system.
The radio access network covers a geographical area which is divided into cell areas. Each cell area is served by a base station. In some radio access networks, the base station is also called e.g. Radio Base Station (RBS), evolved NodeB (eNB), NodeB or B node. A cell is a geographical area where radio coverage is provided by the base station at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. The base station communicates over an air interface operating on radio frequencies with the user equipment within range of the base station.
A current Channel Quality Indicator (CQI) of the user equipment is provided to the radio access network. This CQI value is then used by the base station to calculate the amount of data that should be sent to the user equipment in the next transmission. The system may recover fast from errors by using Hybrid Automatic Repeat reQuest (HARQ). HARQ is a technique that enables faster recovery from errors in communications systems by storing corrupted packets in the receiving device rather than discarding them. Even if retransmitted packets have errors, a good packet may be derived from the combination of bad ones.
Multiple Input Multiple Output (MIMO) refers to any communications system with multiple antennas at the transmitter and/or the receiver, and it is used to improve communication performance. The terms input and output refer to the radio channel carrying the signal, not to the devices having antennas. At the transmitter (Tx), multiple antennas may be used to mitigate the effects of fading via transmit diversity and to increase throughput via spatial division multiple access. At the receiver (Rx), multiple antennas may be used for receiver combining which provides diversity and for combining gains. If multiple antennas are available at both the transmitter and receiver, then different data streams may be transmitted from each antenna with each data stream carrying different information but using the same frequency resources. For example, using two transmit antennas, one may transmit two separate data streams. At the receiver, multiple antennas are required to demodulate the data streams based on their spatial characteristics. In general, the required minimum number of receiver antennas is equal to the number of separate data streams. 4×4 MIMO, also referred to as four branch MIMO, may support up to four data streams. In general, MIMO may be n×n MIMO, where n is the number of antennas and is positive integer. For example 2×2 MIMO, 8×8 MIMO, 16×16 MIMO etc.
Some terms will now be explained. A transport block holds the data that is going to be transmitted, and the transport block is converted into a codeword. A codeword may be defined as the number of transport blocks which have the same HARQ-process identifier. A codeword may be mapped to a number of layers. The term “layer” is synonymous with “stream.” For MIMO, at least two layers are used. The number of layers is always less than or equal to the number of antennas. Precoding modifies the layer signals before transmission. A transmission rank refers to the number of transmitted data streams.
Channel feedback information, also referred to as Channel State Information (CSI), enables a scheduler to decide which user equipments that should be served in parallel. The user equipment is configured to send at least one of the following three types of channel feedback information: a CQI, a Rank Indicator (RI) and a Pre-coding Matrix Indicator (PMI). CQI is an important part of channel information feedback. The CQI provides the base station with information about link adaptation parameters which the user equipment supports at the time. The CQI is utilized to determine the coding rate and modulation alphabet, as well as the number of spatially multiplexed data streams. RI is the user equipment recommendation for the number of layers, i.e. the number of data streams to be used in spatial multiplexing. RI is only reported when the user equipment operates in MIMO mode with spatial multiplexing. The RI may have the values 1 or 2 in a 2×2 MIMO configuration i.e. one or two transmitted data streams. The RI may have the values from 1 and up to 4 in a 4×4 MIMO configuration. The RI is associated with a CQI report. This means that the CQI is calculated assuming a particular RI value. The RI typically varies more slowly than the CQI. PMI provides information about a preferred pre-coding matrix in a codebook based pre-coding. PMI is only reported when the user equipment operates in MIMO mode. The number of pre-coding matrices in the codebook is dependent on the number of antenna ports on the base station. For example, four antenna ports enables up to 64 matrices dependent on the RI and the user equipment capability. The PMI indicates a specific pre-coding vector that is applied to the transmit signal at the base station.
In a common approach to link adaptation for wireless communications, the transmitter adjusts one or more transmission parameters responsive to changes in the receiver's channel quality. The receiver supports link adaptation by the transmitter by sending channel quality information as feedback to the transmitter. For example, the receiver periodically or aperiodically measures channel quality and sends corresponding CQIs to the transmitter, which uses the reported CQIs to adjust the modulation and coding scheme used for transmitting to the receiver.
Ongoing signal quality measurements at the receiver drive CQI generation and feedback. For example, the receiver periodically or aperiodically measures received signal quality as a signal-to-noise ratio (SNR), and maps the measured SNRs into a defined table of CQI values, each value representing a range of SNRs in dBs. CQI may be expressed in terms of transport format sizes which approximately follow an SNR dB scale. Here, the receiver estimates the largest transport format that can be received at a defined reliability or other performance metric. In such embodiments, the CQI values quantize measured SNR and provide a more compact signalling format, which is desirable for high CQI reporting rates. Of course, CQIs can be based on measures other than an SNR scale. Regardless, higher CQI reporting rates are used in more sophisticated wireless communication networks to drive fast dynamic scheduling and link adaptation, which allows those systems to achieve high bit rates and high system throughput.
Even so, it is known in the art to mitigate an “aging” problem. For example, patent application published as US 2005/0181811 A1 teaches “correcting” CQI feedback from a receiver according to an “offset” value. As this reference explains, a channel-dependent scheduler at a base station schedules the user or users reporting the best channel conditions, but the actual channel qualities for those users may have deteriorated by the time the scheduled transmissions occur. The reference thus looks at additional information that can be used to get a more accurate sense of channel quality. In one embodiment, ACK/NACK feedback from a receiver provides a basis for determining or otherwise updating an offset value that is used to correct CQI feedback from the receiver. In this manner, CQIs reported by the receiver can be adjusted by being increased or reduced by a performance-based offset that is determined by monitoring one or more parameters indicative of reception performance. The approach is useful in that it helps prevent the selection of overly optimistic transmission parameter settings.
Another known mitigation technique applies a similar type of offset to reported CQIs, but bases the offset on CQI age. The published international patent application WO 2006/075208 A1 provides an example of age-based CQI compensation in the HSDPA context, where it is suggested that applying corrective back-off or offset values to all CQIs is less preferable than applying an age-dependent offset, in the sense that a relatively new CQI may well provide an accurate sense of current channel conditions at the reporting receiver. It further teaches applying an offset to reported CQIs, where the magnitude of the applied offset is determined as a function of CQI age.
Neither of the above approaches directly addresses the challenges posed by some of the newer communication network standards, such as Long Term Evolution (LTE). Like HSDPA and other high-rate services, LTE relies on fast link adaptation and dynamic user scheduling to achieve high bit rates and maintain high data throughput. For example, an LTE base station, referred to as an eNodeB, may perform link adaptations on a one millisecond basis. LTE receivers support such operations by generating periodic CQI reports according to measurements taken from common reference symbols received in the downlink. The receivers send CQI reports on a physical uplink control channel (the PUCCH, for example), and also may send CQI reports on a physical uplink shared channel (the PUSCH, for example), responsive to receiving grants from an eNodeB.
Problematically, however, certain modes of operation in LTE can result in significantly extended delays between CQI reports from a given user, as compared to HSDPA, for example. In the current LTE standards, the reporting delays for CQIs may be as short as four milliseconds, and as long as eighty milliseconds. Such variability significantly complicates any approach to CQI correction, as there may not be enough recent feedback for performance-based back-offs. Further, with the wide variability in reporting delays and the potential for very long reporting delays, the known approaches to age-based back-offs may produce overly conservative back-offs, which lowers data throughput below achievable levels and thus wastes link capacity. Published patent application US 2012/039207 A1 teaches that reported channel quality information, as used for controlling one or more aspects of wireless transmission, is compensated according to an aging function that depends on channel variability. In this manner, the “amount” or extent of age-based compensation applied to the channel quality feedback for a given user—e.g., a mobile station or other item of user equipment—varies as a function of that user's channel conditions. More particularly, the aging function applied to the channel quality estimates received from (or generated for) a given user depends on estimates of that user's channel variability. Channel quality estimates for a user whose channel conditions are changing very little, or at least are changing very slowly, may be aged less aggressively than those associated with a user whose channel conditions are changing more rapidly. The uplink feedback for support of downlink data transmission comprises the RI, the PMI, and the CQI. The CSI reporting is an important feature to report the channel status from UE to network in order to enable the link adaptation with radio resource scheduling to optimize the system capacity. The physical channels that may be used for the uplink feedback signalling are Physical Uplink Control Channel (PUCCH) and Physical Uplink Shared Channel (PUSCH).
Lack of unified approach for applying link adaptation in different UEs may degrade overall system performance, i.e. sub-optimizing overall throughput in the communication system. It is therefore a desire to provide approaches for limiting such sub-optimization.